Cats are territorial, so the constant weeing and pooing that they do serves to mark territory for other cats, to warn them off. If another cat fancies it chanes, it will mark as well, and so begins a war of attrition - and mess and odour for you.
You need to break the cycle, which is based on scent, so eradication of odour is vital.
Any faeces in the soil must be removed, and the soil under it taken as well. Any faeces on concrete must be removed, and along with concrete areas, must be scrubbed down thoroughly with a stiff yard brush and a strong cleaning solution. Once done, repeat - cats have sensitive noses, so you must remove all traces.
Now get a few boxes of moth balls. Spread liberally whereever the cats like to perform, but also scatter some around your boundaries, Include the tops of walls, however nigh, and fences, in fact anywhere that's an edge to your property. The whiff is a bit pungent, but it will keep the cats away. Maintain the mothballs which eventually melt in rain, and you should have no further problems.
Good luck.